1. Field
The subject invention is in the field of mechanical power transmission and transfer mechanisms, particularly the field of power transfer from an engine in a boat to the propeller driven by the engine. More particularly it is in the field of such drives in which the drive shaft and propeller shaft are parallel and essentially horizontal such as in well known inboard/outboard drives in which power is transmitted through the stern of a boat above the waterline and then down to the propeller shaft and propeller. However, the subject drive is an inboard drive which transmits power through the bottom of the boat. A feature common to inboard/outboard drives and similar inboard drives is that the drive shaft and propeller shaft are parallel and power is transmitted between the two using bevel and/or miter gearing, chains or belts. One important objective of such drives is that the components in the water present as low frontal area as possible to minimize drag losses. This is particularly important for sailboats in which the propeller is an auxiliary power source and must present minimum drag when the boat is under sail. The problem is more severe for larger boats in which auxiliary power levels are in the range of 100 to 200 H.P. Since such boats are not high speed boats, propeller speeds must be relatively low and propellers fairly large to achieve satisfactory efficiency. These facts require that the torque capacity of the drive be high relative to the horsepower level. In the stated horsepower range high torque per horsepower gear drives become bulky and require undesirably large frontal areas under water and, for assembly reasons, their casings comprise several parts, in many cases having long parting lines. Chain drives are better suited to high torque per horsepower transmissions; however good operation, efficiency and long life of chain drives, particularly bi-directional drives, requires that the chains be under tension and correspondingly free of slack and running in a straight line from sprocket to sprocket. It is close to physically impossible and economically and practically impossible, using conventional techniques, to design and assemble a chain drive in which the chain is always in tension without using some sort of tensioning device. This is caused in large part by the fact that use invariably involves wearing in and wear which allow the chain(s) to go slack. Tensioning devices inherently tend to add undesirable amounts of frontal area and complication.
2. Prior Art
There is much prior art in the particular field described above and many of the prior art drives use chains. The U.S. patents listed here are typical examples:
As background to discussion of this prior art, it is important to state that the chains having the highest power capacity for their size and weight are chains known as silent chains. These chains comprise pluralities of flat links having a tooth form at each end. The side-by-side links are pinned end to end so that the teeth forms form teeth when the chain is in contact with the sprockets on which it is mounted. Belt width is determined by the numbers of links pinned side-by-side. Making and using these chains as mechanically efficiently as possible resulted in their having the characteristics that (1) they allow only limited bending in the direction away from the toothed side of the belt, and (2) the durability and efficiency depend on their being as straight as possible between sprockets at all times. These factors relate to the need for tension adjustment and prohibit techniques using tension idlers which would not allow the chain to be straight between sprockets.
Regarding the cited prior art, patents 2, 5, 6, 7, 8, 9 and 10 utilize roller chains and show no specific means for adjusting tension except in patent 9. The means shown in patent 9 comprises an oval shaped cam pivotally mounted midway between the lengths of chain between sprockets so that rotating the cam in one direction so that the cam ends contact the chain lengths spreads them apart to increase tension. This does not allow the chain in tension to be straight. Also, this means can only be used in unidirectional drives. With rotation in the wrong direction the cam would be forcefully rotated into the chain and jammed.
The remaining patents show drives using belts of some kind. Patent 1) uses a toothed belt and shows no means of adjusting tension even for the purpose of removing and installing the belt. Patent 3) also shows the use of a toothed belt with means for adjusting tension for the purpose of removing and installing the belt but none for compensating for belt stretch and other factors which are known to cause loosening of toothed belts. The means used comprise a spherically mounted bearing on one end of the drive shaft so that when the housing is disassembled and the bearing at the other end of the drive shaft is removed, the shaft can drop to an angle sufficient to allow the teeth on the belt to clear the rim on the sprocket, thus facilitating removal and replacement. The end of the shaft freed by dismantling the casing is tapered to facilitate its reentry into the bearing when the case is assembled, leveling the drive shaft again and providing nominally acceptable belt tension. However, this adjustment feature does not allow compensation for belt stretch and other factors which are known to cause belt loosening. Also, to enable replacement of this belt the casing is divided vertically into forward and aft parts, generating a need for long parting surfaces and a plurality of fasteners are needed to assemble the casing.
Patent 4) shows the use of dual toothed belts. There are no provisions for tension adjustment and the method of assembly and disassembly of the belt drive is not disclosed.
Many motorcycles transmit driving power from a drive shaft to a driven shaft, the rear axle. Chain tension is adjusted by adjusting the position of the rear axle and everything carried by it relative to the drive shaft. This technique cannot be used in propeller drives because the driven shaft must be enclosed in a housing.
In view of this prior art, the objective of the subject invention is to provide a low frontal area, inboard, through-hull propeller drive for power ranging up to 200 H.P. at maximum propeller shaft speeds of 2500 RPM, the drive using a silent chain and having (1) a housing having a minimum number of parts and short parting lines, (2) simple means for adjusting chain tension for installation and removal purposes and compensation for wear, and (3) allowing simple chain installation and removal.
The subject invention is a low frontal area, inboard, through-hull propeller drive. The drive comprises (1) a main casing having a high fineness ratio streamlined cross section shape strut and a propeller shaft, chain sprocket and bearings installed in its lower end through the opening for the propeller shaft and its bearings, (2) a sub casing attached to the upper end of the main casing, comprising upper and lower halves and having a drive shaft, sprocket and bearings installed in it, and (3) a silent chain interconnecting the sprockets which are of the same diameter. There is a plurality of chain tensioning screws in the bottom half of the sub housing which are used to adjust the distance of the sub housing from the main housing, thus adjusting chain tension. There is a sealing system in the bottom surface of the bottom half of the sub housing which seals the gap between the main and sub housings independent of the width of the gap between the two. The housing and sub housing are interconnected by a plurality of bolts.
The chain is installed with the top half of the sub housing removed. An end of the disconnected chain is passed into the main housing strut around the propeller shaft sprocket and through the main housing strut and out through its top end so that both ends of the disconnected chain are accessible. The ends of the chain are passed through a hole in the bottom of the bottom half of the sub housing and it is placed on the top of the main housing. The drive shaft with the sprocket in place is then placed in the bottom half of the sub housing with no bearings installed and the ends of the chain are connected. The absence of the top half of the sub housing allows ample accessibility for connecting the chain ends and the absence of the bearings allows ample slack in the chain for connection purposes. The chain is connected by pins as long as the width of the chain so clearance space at least equal to the width of the chain must be available off one edge of the chain. If two chains are used, clearance must be available off both outer edges. Total chain widths up to eight inches are anticipated. The bearings are then put in place, first the inner races, with the rollers and spacers and then the outer races. Two rings are then set into circumferential slots in the half bore of the bottom half of the sub housing to retain the bearings against the ends of the hub of the sprocket and to position the drive shaft and sprocket longitudinally. This procedure positions the drive shaft properly and takes up most of the slack in the chain. There are four tensioning screws in holes in the sub housing in contact with pressure bars which distribute and transfer tensioning forces to the top plate of the main housing. Turning these screws inward moves the bottom half of the sub housing away from the main housing, increasing the distance between the drive and propeller shafts and putting the chain in tension. This adjustment can be used at any time in the life of the drive to maintain adequate tension in the chain.
The top half of the sub housing is then put in place and bolts are installed through it and the bottom half to attach them to the main housing. Caps are then inserted into the ends of the bore around the drive shaft ends. The caps hold seals which prevent oil leakage around the drive shaft. A sealing system prevents leakage of the oil, with which the drive is filled, through the gap between the sub and main housings. The sealing system comprises a rectangular cross section groove in the bottom of the bottom half of the sub housing. The groove surrounds the holes for the chain in the top plate of the main housing and the bottom half of the sub housing. An elastomeric seal ring having a cross section size and shape such that it fits closely in the groove is installed in the groove. A grease fitting is installed in the bottom half of the sub housing and connected by drilled holes to the groove. Forcing grease into the groove presses the seal ring against the sides of the groove and the top plate, thus sealing the gap. This seal is re-pressurized after each adjustment of chain tension. Both ends of the drive shaft extend beyond the sub housing so that power can be applied to either end.
The plate on the top of the strut extends beyond the housing and strut to be attached to hull structure to mount the drive.
The invention is described in more detail below with reference to the attached drawings.